1. Field of the Invention
The present invention is generally in the field of semiconductor fabrication. More particularly, the invention is in the field of fabricating emitter structures in transistors.
2. Background Art
In silicon-germanium (“SiGe”) heterojunction bipolar transistor (“HBT”) technology, one approach to formation of an emitter in the SiGe HBT includes filling an emitter window opening with polysilicon material. One conventional method of filling the emitter window opening with polysilicon material comprises depositing undoped polysilicon into the emitter window opening, implant doping the polysilicon, and activating the dopants in the polysilicon.
Disadvantageously, the aforementioned method for fabricating SiGe HBTs can produce transistors having different characteristics depending on emitter window widths. Polysilicon deposits relatively conformally within an emitter window opening having a wide emitter window width. However, an unwanted “plug effect” can occur for emitters having narrow emitter window widths. The plug effect refers to non-conformal deposition of undoped polysilicon within a narrow emitter window, resulting in a higher thickness of polysilicon in the center of the emitter window relative to the emitter window sidewalls; hence resembling a “plug” in the narrow emitter window. The plug effect, combined with implant doping of the emitter, also result in a difference in the doping profile in the emitter and base of transistors having narrow emitter windows relative to transistors having wide emitter windows.
Another disadvantage of the conventional methods occurs due to the implant doped polysilicon being in direct contact with the base. Activating dopants in the implant doped polysilicon produces a difficult-to-control diffusion into the base, which degrades HBT performance.
Therefore, a need exists for fabricating transistors, such as SiGe HBTs, having substantially similar characteristics regardless of the emitter window width, and for controlled diffusion of dopants into the base, to achieve improved performance.